1. Field of the Invention
This invention relates to heteroepitaxial growth with minimization of the effect of crystallographic misfits.
2. Brief Description of the Prior Art
Heteroepitaxy on substrates with large misfits results in a large number of defects, such as threading dislocations. Therefore, for materials that require a low defect density, only substrates with a small misfit can be used for heteroepitaxy. This requirement severely limits the choice of substrates.
Heteroepitaxy allows different materials to be combined and allows properties of each to be simultaneously exploited. Some commercially important examples of heteroepitaxy include germanium or gallium arsenide on silicon, or silicon on sapphire. Heteroepitaxy generally generates defects and, in general, the number of defects is dependent upon the magnitude of misfit between the film and the substrate. The properties of semiconductors generally degrade rapidly with increasing density of defects, therefore only low misfit substrates can be used to grow semiconductors.
Silicon carbide is a semiconductor material that displays great potential for high temperature and high power microwave properties. Homoepitaxy on 6H--SiC (a hexagonal crystal structure) substrates has previously been the only way to make good quality SiC for devices. Heteroepitaxy of 3C--SiC (a cubic crystal structure) on Si has been successful but the defect density is too large for device applications and the Si substrate has many other serious limitations in terms of device performance, such as low resistivity and low melting point (compared to standard SiC growth temperatures). The mismatch between Si and SiC is about 20%. Sapphire (Al.sub.2 O.sub.3) is another substrate that has much better properties than Si. The misfit between sapphire and SiC is still very large, about 13%. Aluminum nitride and gallium nitride have similar structures and lattice parameters to SiC and these materials grow epitaxially on sapphire, but again with a large density of threading dislocations. The microstructure of gallium nitride on sapphire, in general, consists of regions of very low density of threading dislocations surrounded by areas of very high density of threading dislocations (See Z. Sitar et al., "Growth of AlN/GaN Layered Structures by Gas Source Molecular-beam Epitaxy", Journal of Vacuum Science Technology (1990). This microstructure results from the initial three-dimensional growth of the film. The crystallographic axis of each island is rotated slightly with respect to the substrate. When these islands grow together and form a continuous film, the misalignment between the island and the substrate becomes misalignment between neighboring grains with low angle grain boundaries to accommodate this misalignment. The degree of misalignment is primarily related to the misfit, but growth conditions also influence the final threading dislocation density greatly.
The best quality gallium nitride on sapphire has been achieved by a two step growth procedure (See S. Nakamura et al., "High-power GaN P--N Junction Blue-light-emitting Diodes", Japanese Journal of Applied Physics, 30, pp. L1998-2001 (1991) and H. Amano Akasaki et al., "Effects of AlN Buffer Layer on Crystallographic Structure and on Electrical and Optical Properties of GaN and Ga.sub.1-x Al.sub.x N (0&lt;x.ltoreq.0.4) Films Grown on Sapphire Substrate by MOVPE", Journal of Crystal Growth, 98, pp. 209-219 (1989)). The first step is to anneal the substrate at 1050.degree. C. in order to "clean" the sapphire. As will be pointed out below, the initial anneal is extremely important for a different reason. Growth of a thin GaN or AlN buffer layer occurs at a low temperature (600-650.degree. C.). The low temperature results in a very high density of islands and hence a rapid transformation from 3-dimensional to 2-dimensional growth. The growth temperature is then raised to a high temperature and a thick GaN layer is grown. The top GaN has good crystalline quality and a much lower threading dislocation density than those reported by other growth techniques. The high temperature growth results in the gradual reduction and even cancellation of defects which were formed in the initial buffer layer. The primary disadvantage of this technique is that, while it improves the defect density, there still remains a very high level of defects.
While the large misfit between substrate and film results in a very large misfit dislocation density, these dislocations are localized near the interface and hence do not degrade the performance of the devices. The "bad" dislocations are the threading dislocations. In large misfit systems, it is much more difficult to form threading dislocations inside islands because the critical thickness is so small that the dislocations are introduced almost immediately. This observation helps to explain the microstructure of GaN on sapphire. Threading dislocations in low misfit systems generally form by interactions between misfit dislocations while they are moving to accommodate the stress during growth. This interaction happens after the film has started to grow two-dimensionally. What is needed is a method to align the islands to the substrates such that the misalignment between neighboring islands is much smaller. It is the misalignment between islands which generally results in the large defect density in large misfit heteroepitaxy and not the misfit dislocations themselves.